Back to the OP's point, while a 400mm prime can be used for landscapes, it's likely not ideal. I and others have mentioned using a 70-xxx or 100-400 zoom for landscape shooting, but how much of that is at the long end? For me, many are in the 70-150mm range. The 100L Macro also makes a very nice landscape lens...

It can be difficult to use a long telephoto for landscapes. The issues can sometimes be overcome.1. Haze in the air shows up easily, so you must time your shots to minimize it, such as early mornings.

2. Heated air between you and the distant subject becomes more of a issue, once again, the solution is cold days or early mornings.

3. Just the amount of air between you and the subject means that the resolution is going to suffer, so don't expect high resolution.

In some cases. its the only option, but I would not want to try it on a regular basis.

Here is one image of the Alki Point Lighthouse in West Seattle taken from a moving tour boat at 235mm with my 7D and 100-400mm L on a cold and hazy day. I boosted the blues in pp perhaps too much because of all the gray.

I also tried to get a shot of Mt Rainier, but the hade overwhelmed it. Even trying to bring it out in PP didn't reveal much. Once again, the boosted blues to help the gray sky just colored the haze blue. The Mt Rainier image was at 300mm.

...

The Mt Rainier shot could benefit from the "Lumosity Blend" technique Kieth goes over at NL to make the mountain stand out without boosting the sky or overly altering the color.

If you don't mind me posting an edit of your image I can show an example.

I don't have time to work on it, but feel free to improve it, I'm sure its possible. It is probably best to start with the original. I can upload the raw if you'd like, here is a jpeg. As you can see, there isn't much detail in the mountain.

Back to the OP's point, while a 400mm prime can be used for landscapes, it's likely not ideal. I and others have mentioned using a 70-xxx or 100-400 zoom for landscape shooting, but how much of that is at the long end? For me, many are in the 70-150mm range. The 100L Macro also makes a very nice landscape lens...

I find that medium telephoto such as the 135 f2 and 200 2.8 can be useful landscape lenses, but even with this focal length it is very easy to suffer from the problems listed by others; shake, air diffusion and dof being the main ones. I do like the potential of the tight framing and compression though. I even use them for panos, the picture below of Whitby Abbey was shot on the 135L, but as I was shooting in sections and pretty close diffusion wasn't an issue despite it being very early in the morning ( 5 o'clock ) and a light sea mist.

In the bottom picture, which is a single frame, you can see how the line of trees behind the bracken is very sharp but the hills beyond are softer due to air diffusion. Actually I think it produces a pleasant effect.

Personally I wouldn't want to use anything longer than 200mm in this way.

Back to the OP's point, while a 400mm prime can be used for landscapes, it's likely not ideal. I and others have mentioned using a 70-xxx or 100-400 zoom for landscape shooting, but how much of that is at the long end? For me, many are in the 70-150mm range. The 100L Macro also makes a very nice landscape lens...